1. Field of the Invention
The present invention concerns position encoders and in particular incremental encoders having a zero mark signal.
2. Background Art
Position encoders ("encoders") are electro-mechanical devices producing a digital output related to the position of a movable element of the encoder. In one such encoder design, a rotary encoder, the movable element is a rotatable shaft. The rotatable shaft is attached to a disk shaped rotor having an optically readable pattern marked on its surface. The pattern may be formed by alternating opaque and transmissive frames. The frames are illuminated from one side by a illumination source and light travels from the illumination source through the opaque and transmissive frames of the rotor and similar frames in a stationary stator mask. The light transmitted by the stationary mask is detected by a stationary photodetector. Rotation of the shaft moves the rotor which in turn causes a fluctuation in the transmitted light producing a signal that may be decoded into a digital indication of shaft movement.
Encoders may be classified as absolute encoders or incremental encoders. Absolute encoders produce a unique digital code word for each encoder position. The rotor of an absolute encoder may carry a multi-bit Grey code, for example, where each bit is read by a separate photodetector to produce the output digital word.
Incremental encoders provide only an indication of the change in position. The rotor of an incremental encoder ordinarily contains a uniform periodic pattern whose movement past a photodetector creates an increment signal indicative the amount that the shaft has rotated. Two or more photodetectors arranged with an offset of 90.degree. ("quadrature") may be used to provide an indication of the direction as well as amount of rotation of the shaft, as is understood in the art.
Frequently, a zero mark track is included on the rotor of an incremental encoder. The zero mark track provides a reference signal that indicates that the encoder shaft is at a "zero" position. This zero mark signal typically resets a counter that counts up or down with the increment signals from the encoder. The value of the counter thereby provides a running indication of the absolute encoder position.
The zero mark signal may be generated by the alignment of a single transmissive frame of the rotor with a corresponding frame in the stationary mask ("stator mask.revreaction.) to allow light from an opposed source of illumination to strike the surface of a photodetector. The position of this alignment is defined to be the zero position for the encoder.
The drawback to the use of a single transmissive frame for generating the zero mark signal is that, as the resolution of the encoder increases, the size of the single frame becomes increasingly small. A reduction in area of the transmissive frame decreases the light detected by the photodetector causing a decrease in the signal-to-noise ratio ("SNR") of the zero mark signal.
Two approaches have developed to counteract the decreasing SNR of the zero mark signal with increasing encoder resolution. In one approach, miniature lenses are used to concentrate the light transmitted by the single transmissive frame more efficiently on the photodetector. Alternatively, multiple, staggered transmissive frames have been used to generate the zero mark signal. Together, the area of the staggered frames is large, improving the zero mark SNR, and yet the area of each frame is small preserving the resolution of the zero mark signal. At the zero position of the encoder, the staggered frames on the rotor match corresponding frames on the stator mask to transmit light. Careful choice of the frame spacing can be used to minimize the transmitted light at the non-zero positions of the encoder.
This technique is described in U.S. Pat. No. 3,187,187 to Wingate and entitled "Photoelectric Shaft Encoder". Wingate describes several patterns of transmissive elements used to produce a zero mark signal. The patterns are derived from short arithmetic progressions that reduce the matching of transmissive segments of the rotor and stator mask for nonzero encoder positions.
Similar patterns of greater length, also based on arithmetic progressions, are taught by LaPlante in U.S. Pat. Nos. 4,602,155 and 4,678,908.
These techniques for generating the rotor and stator patterns are cumbersome for use in high resolution encoders where long zero mark patterns are desired. Further, such arithmetic progressions may produce zero mark signals with substantially less than optimum noise immunity.